The Tomato Magazine

The Tomato Magazine
December 2006

Purple Tomato Releases Expected Soon

Purple tomatoes, rich with health-promoting compounds, may soon be available to commercial and home gardeners. Oregon State University plant breeders hope to release the first in a series of purple-skinned varieties in approximately two years, according to Dr. Jim Myers, a professor of vegetable breeding
and genetics at Oregon State University, Corvallis, Ore.

First on the market will be saladette or smaller sized slicer tomatoes, the researcher says. Crosses also are underway to come up with purple-colored cherry and larger slicer and processing-types.

Began as a Research Project
History of OSU’s purple tomato work dates back to 2000, when Myers okayed graduate student Carl Jones’ request to investigate what could be done to enhance the health benefi ts of certain vegetables. The decision was made to focus on tomatoes.

“At the time, we were looking primarily at carotenoids and at some of the different genetic mutants that were available in tomato,” Myers recalls. “By and by, Jones discovered a particular gene, the anthocyanin fruit gene in tomatoes that produced a purple fruit.”

The discovery was especially exciting due to the reported health benefits of certain anthocyanin-rich foods, such as blueberries and red grapes processed to produce wine. and therapeutic roles in a number of human diseases. Through the much publicized ‘French paradox,’ the public has become aware that certain populations of red-wine drinkers in France and Italy have much lower rates of coronary heart disease (CHD) than their North American and Northern European counterparts. It is widely accepted that red wine phenolics contribute at least partly to this beneficial effect.

“The anthocyanin pigments of bilberries (Vaccinium myrtillus) have long been used for improving visual acuity and treating circulatory disorders,” Wrolstad continues. “There is experimental evidence that certain anthocyanins and fl avonoids have anti-infl ammatory properties, and there are reports that orally
administered anthocyanins are benefi cial for treating diabetes and ulcers and may have antiviral and antimicrobial activities. The chemical basis for these desirable properties of fl avonoids is believed to be related to their antioxidant capacity—their ability to scavenge (clean away) and trap free radicals that damage biomolecules.”

Health Compounds
The specific anthocyanins present in OSU’s purple tomatoes are mainly petunidin, but malvidin and delphinidin are also present, Myers explains. The anthocyanins are modifi ed by the presence of acyl (sugar) groups. Anthocyanins are a member of a larger class of compounds called fl avonoids. Other members of this class include quercetin, kaempferol, naringenin, catechin and isolfl avones.

Phenols or phenolics are related compounds that differ in basic chemical structure but have similar function. Although the current popular theory is that the health benefi ts of anthocyanins are due to their antioxidant activity, recent research suggests that anthocyanins may not act directly as antioxidants. The
thought is that they may produce health benefi ts in a more complex manner, Myers notes.

Once Jones had completed his graduate work and moved on, a second graduate student, Peter Mes, continued on under Myers’ direction. Mes focused on trying to increase the intensity of the purple pigment in the fruit. While doing so, he obtained other genetic mutants that produced small quantities of anthocyanin.

From there, he and Myers concluded that they could combine these particular genes and come up with a much more intense color. In fact, the fruit directly exposed to sunlight throughout the growing season developed skins that were as dark purple as eggplant.

“These genes originate from a wild species,” Myers says.“Anthocyanin fruit is derived from a wild species, Solanum chilense.“The material we used originates
from a genetic stocks collection at the University of California-Davis. It was first developed by a Russian researcher who back crossed it into cultivated
tomatoes from a wild species.”

There are a few quirks with consistency of color yet to be worked out, Myers acknowledges.

“The purple pigment is only expressed in those portions of the fruit that are exposed to sun light,” he details. “Where the fruit is shaded by a leaf or the calyx, or on the base of the fruit, when the fruit ripens, it will turn a normal red color. We’d like to come up with types that exhibit purple skin all of the time—no matter what the light conditions are.”

The other negative is the purple pigment is expressed only on the skin. The interior of the fruit is red, Myers says. The hope is to come up with a purple-fl eshed interior.

Greater Potential Benefits
While not there yet, the university’s purple-skinned tomatoes do have anthocyanin. While the levels are about 1/10 to 1/20 of that found in blueberries—blueberries are one of the richest sources of anthocyanins known—Americans consume far more tomatoes per year than blueberries. The potential health benefi ts could be significant.

“In recent years, per capita consumption of tomatoes in the U.S. is about 90 pounds per person per year,” Myers points out. “In contrast, consumption of fresh and frozen blueberries per person per year averages less than one pound.”

Does the presence of anthocyanin affect the taste of the potential purple tomato releases?

“ Some of the lines with extremely high anthocyanin have a strong flavor described as ‘inky,’” admits Myers’ current graduate student, Peter Boches, adding that “Pure anthocyanin has no flavor, so this may be a result of related changes in chemical composition. We have evidence to suggest that at least one of the
genes involved is a regulatory gene that interacts with several biosynthetic pathways; so multiple changes in the chemical composition of the tomato may have taken place in order to accommodate the increased anthocyanin production, he explains.

“The taste of tomatoes in the breeding program could also be influenced by the presence of other ‘wild’ genes that were carried along on the chromosome with the genes that activate anthocyanin production,” the researcher theorizes. “By crossing extremely high anthocyanin lines with tomato cultivars known for
their good taste, such as ‘Sungold,’ we have recovered individual plants that produce tomatoes with high amounts of anthocyanin and acceptable flavor.
We anticipate being able to produce stable lines with good fl avor and moderate amounts of anthocyanin from these individual plants or others like them.”

Meanwhile, the OSU professor is confident that his graduate students have come up with a purple tomato line that is distinctly different and has more to
offer than ‘Black Prince,’ ‘Purple Cherokee’ and other so-called purple lines already on the market.

“All of the ‘black’ and ‘purple’ varieties that we have examined in our fi eld trials have the green flesh gene which prevents normal chlorophyll breakdown,
resulting in a brown compound called pheophytin,” Myers points out. “In combination with pink or red color from the carotenoids normally found in tomato
fruit, a muddy brownish purple color is produced.The biochemical composition of the tomatoes that we have developed is fundamentally different.”

As for now, priority No. 1 is to release “a bunch” of varieties within the next two years, Myers says. The first releases will be small slicer and saladette types for the fresh market. Next will be purple cherry tomato and larger slicer and processing types to provide offerings for virtually every need.

Will purple tomatoes prove a hit with the public? That remains to be seen, the researcher admits. Since news of the research at OSU hit the press, however, inquiries have been pouring in from all over the world. While most of the interest has come from home gardeners, feedback also is coming in from fresh and processed markets.

Funding for the OSU purple-tomato breeding program is provided through an endowed chair. This has enabled Myers and his graduate students to move forward without worrying about how to cover their research costs.

Since the OSU tomato breeding program got under way in the 1950s, about 13 varieties have been released to the public, Myers says. One of the better known varieties is Legend, a very early, large late blight-resistant slicer type that has done well under Oregon climatic conditions.

Editor’s note: Jim Myers can be contacted at myersja@hort.oregonstate.edu.

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